Conventional heart-rate sensors are embedded in monitors and are commonly used in training and sports. There are two common types of heart-rate sensors:
The first type: is a monitor comprising a chest strap sensor and a receiver integrated into a wristwatch. The strap receives ECG signals from the heart and transmits them to the receiver. An example for this type is RS400 Heart Rate Monitor (manufactured by Polar). The chest belt receives ECG signals from the objects body chest via two electrical contacts, and transmits them to the receiver in the wristwatch.
The chest strap associated with this type is inconvenient to wear. Also, in some individuals, the ECG signal reception is too week, due to the varying body surface resistance. This may result in intermittent pulse rate measurements. This monitor type is the only practical solution currently available for continuous heart rate measurement during exercise.
The second type: is a beltless heart rate monitor where an ECG sensor is integrated in a wristwatch. The ECG signals are conducted to the measurement device inside the watch via the two subject's arms. It requires the user to close an electrical circuit by touching the watch with his other hand. An example of this product is the PM18 by Beurer GmbH. However, for continuous pulse rate readout during training, it is impractical to use the beltless solution, as it requires the continuous connection of both hands.
Another type of heart-rate monitor available on the market is ring-shaped, like the Multifunction Digital Ring by Lifespan. This monitor is worn on the finger, utilizing a sensor that measures the blood pulsating in the finger, using infrared imaging. However, this measurement is not sufficiently accurate, especially while in training.
U.S. Pat. No. 4,085,740 discloses heart-rate measurement using microwave sensors. Also, James C. Lin, in his paper titled “Microwave Apexcardiography” in Transactions On Microwave Theory And Techniques, Vol. Mtt-27, No. 6, June 1979 teaches a measurement method of the heart rate, directly from the heart movement. Lin utilized a quadrature homodyne detector to detect the amplitude and phase of the reflected signal from the heart.
U.S. Pat. No. 4,958,638 teaches a method to measure the heart-rate and respiratory rate by directly measuring the heart and lungs movements. This method is able to distinguish between movements of the two. This patent describes a frequency modulated radar, which can be interpreted as FMCW as suggested by US 2010/0179438. However, both patents do not deal with the problem of separating the heart pulse rate measurement from other body organ movements. Such movements frequency, in the case of sport training, fall within the normal heart rate frequency range and are not easy to filter out.
Attempts have been made to filter this interference, for example in U.S. Pat. No. 5,807,267 that suggests using an additional accelerometer to measure the body movement frequency and cancel it, using an FFT (Fast Fourier Transform) technique. FFT for this application implies a long measurement time for reasonable accuracy, which cannot be tolerated in this type of application. Another example of this technique is described in a promotional activity by Epson describing a prototype E200 pulse watch, for example as described in their internet site at http://global.epson.com/innovation/technology_articles/201206_2.html.
All the sensors described above have not provided a satisfactory solution to the problem of the problem of separating the heart pulse rate measurement from other body organ movements and are not sufficiently accurate.
It is therefore an object of the present invention to provide a wireless heart-rate sensor that can separate the heart pulse rate measurement from other body organ movements.
It is another object of the present invention to provide a wireless heart-rate sensor that is accurate, especially for measurements that are made while training.
It is a further object of this invention to facilitate heart rate measurements from a body part which is covered by apparel or by natural fur.
Other objects and advantages of the invention will become apparent as the description proceeds.